Most wooden doors will start exhibiting signs of hollowness when tapped. Usually, termites and other insects are the culprits. Unfortunately, most people don’t notice termites until the door is entirely hollow.
That’s why opting for metal doors or aluminum doors is better in the long run. If you think you might have termites, tap your door with your fingers. If it feels dense and thick, then your wood door is healthy.
However, if the door feels empty, hollow, and limp, you might have a termite infestation. A termite infestation can move to other places of your home and composite the structural integrity. So call a professional door handler so they can properly dispose of and replace your door.